Methods for culturing microorganisms include methods with liquid media and methods with solid media. The methods using liquid media are problematic in that bacterial growth is weak; no mutant strains can be obtained; or only bacteria at low purity can be isolated. Because methods using solid media have fewer such problems by which distinctly separated colonies can be formed, alternatively, the methods are excellent as methods for isolating microorganisms and culturing the microorganisms at high purity. Thus, the methods have been used traditionally. As a solidifying component for the solid media, agar gel has been used traditionally since the era of Dr. Koch. As described in for example “New Experimental Biochemistry Lecture Series No. 17” (edited by the Japan Biochemistry Association, 1992, issued by Tokyo Kagaku Dojin), page 15 to page 20, agar gel is a hard and transparent solid in a wide temperature range where bacteria can grow, so the number of microorganisms decomposing agar is very limited. Thus, agar gel has such excellent properties that the problematic liquefaction scarcely occurs. Accordingly, agar gel is very great as a medium-solidifying agent. It is no exaggeration to say that no excellent medium-solidifying agent replacing the agar gel has been found yet.
However, such agar gel is softened or dissolved, depending on the condition such as temperature pH or salt concentration. Thus, the range where agar gel can be used as a medium-solidifying component is limited to a given condition. So as to screen for new useful microorganisms, recently, it is required to culture microorganisms in a wider range of culture conditions including for example temperature, pH and salt concentration. Therefore, it desired to develop a medium-solidifying component usable in such a wider range of culture conditions. For example, ultra-thermophilic bacteria capable of growing at a temperature as high as 121° C. have been identified so far. Enzymes generated by such ultra-thermophilic bacteria are believed to be industrially useful. However such bacteria cannot be cultured in solid media because the temperature for the growth of such microorganisms is the softening temperature of agar or higher. Therefore, such bacteria have been prepared only by liquid culturing. As described above, however, the growth thereof by liquid culturing is weak; no mutant strains can be obtained; or the purity of the isolated bacteria is low. Compared with the solid culturing thereof, the liquid culturing thereof is significantly limited.
Various attempts have been made to develop a medium-solidifying component usable in a wider range of culture conditions. For example, a method using as a medium-solidifying component gellan gum (see for example Shungu, D., Valiant, M., Tutlane, V., Weniberg, E., Weissberger, B., Koupal, L., Gadebusch, H., and Stapley, E., “Appl. Environ. Microbial.”, 1983, 46, 840-845), as well as a culture process on a silica gel plate containing a medium (see for example New Experimental Biochemistry Series No. 17, page 15 to page 20, edited by the Japan Biochemistry Association, 1992, issued by Tokyo Kagaku Dojin) has been developed. The method using gellan gum is however disadvantageous in that a medium cannot be solidified when cations (metal cations, in the medium are insufficient or in that the increase of the acidity causes poorer solidification of a medium. The method using such silica gel plate is disadvantageous in that the use thereof is tough under alkaline conditions; the resulting plate has a lower water retention capacity and the surface of the plate is readily dried, leading to the occurrence of adverse influences on microbial growth. In other words, it should be said that no solid medium satisfactorily applicable under culture conditions including a wide range of temperature pH and salt concentration has been developed yet.
It is an object of the invention to overcome the problems of related-art solid media such as agar medium sometimes never usable depending on the condition such as temperature, pH or salt concentration as described above and provide a solid medium usable in a wider range of culture conditions and a method for producing the same.